Where are the ionization nebulae predominantly located in the galaxy M51?

within or on the edges of the spiral arms of the galaxy

Why are the ionization nebulae so bright?

They are regions where gas is ionized by hot, young stars.

To calculate the dashed orbits from the stellar positions, astronomers had to assume that __________.

if they observed for many more years, the dots would trace out ellipses

To determine the mass of the central object, we must apply Newton's version of Kepler's third law, which requires knowing the orbital period and average orbital distance (semimajor axis) for at least one star. Let's consider the star with the smallest orbit, which is represented by red and pink dots. What is the approximate orbital period of this star?

13 yr

Look again at the orbit of the star with the smallest orbit, represented by red and pink dots. By comparing the orbit to the scale bar shown on the diagram, you can estimate that this orbit has a semimajor axis of about _____.

800 AU

The following equation, derived from Newton's version of Kepler's third law, allows us to calculate the mass (M) of a central object, in solar masses, from an orbiting object's period (p) in years and semimajor axis (a) in astronomical units:

M=a3p2

Using this formula with the values you found in Parts C and D, what is the approximate mass of the central object?

From Part E you know the mass of the central object. Now consider its size. Based on what you can see in the diagram, you can conclude that the diameter of the central mass is __________.

You've now found that the central object has a mass of about 3 million solar masses but is no more than about 70 AU in diameter—which means it cannot be much larger than the size of our planetary system. Why do these facts lead astronomers to conclude that the central object is a black hole?

What is the typical hydrogen content of stars that are forming right now in the vicinity of the Sun?

What do halo stars do differently from disk stars?

Based on observations, which of the following statements about stars in the Milky Way is generally true?

How do we determine the Milky Way's mass outside the Sun's orbit?

We measure the mass of the black hole at the galactic center from:

How does the diameter of the disk of Milky Way Galaxy compare to its thickness?

What do we call the bright, sphere-shaped region of stars that occupies the central few thousand light-years of the Milky Way Galaxy?

The Sun's location in the Milky Way Galaxy is _________.

What do we mean by the interstellar medium?

What are the Magellanic Clouds?

How do disk stars orbit the center of the galaxy?

How do we know the total mass of the Milky Way Galaxy that is contained within the Sun's orbital path?

Elements heavier than hydrogen and helium constitute about _________ of the mass of the interstellar medium.

What do we mean by the star-gas-star cycle?

What are cosmic rays?

The primary way that we observe the atomic hydrogen that makes up most of the interstellar gas in the Milky Way is with _________.

Which of the following molecules is the most abundant in molecular clouds?

Interstellar dust consists mostly of _________.

Which of the following models best explains why our galaxy has spiral arms?

What do we mean by a protogalactic cloud?

Most stars in the Milky Way's halo are _________.

What is an ionization nebula?

What do halo stars do differently from disk stars?

Where does most star formation occur in the Milky Way Galaxy?

Based on observations, which of the following statements about stars in the Milky Way is generally true?

What kind of object do we think lies in the center of the Milky Way Galaxy?

Sound waves in the interstellar medium

Approximately how far is the Sun from the center of the galaxy?

Where are most heavy elements made?

Compared with stars in the disk, orbits of stars in the halo

Approximately how long does it take the Sun to orbit the center of the Milky Way Galaxy?

What evidence supports the existence of a black hole at the center of our galaxy?

Why do stars in the disk of a spiral galaxy orbit in roughly the same direction, in the same plane in space?

Which star is older?

Open clusters and young stars are generally found only in the disk of the Milky Way and not in the halo.